What to do with spark when checking compression
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From: Live Oak, FL
What to do with spark when checking compression
I just bought a compression tester. The directions say to short out the coil wire to ground and then says if you have electronic ignition to remove the hot wire to the ignition unit. What do you all reccomend I do on my 4.9 inline 6?
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What to do with spark when checking compression
Leave the ignition off. Rig the throttle open. Operate the starter by energizing the starter relay - a screwdriver across the correct two terminals will cause the relay to close and run the starter.
With the ignition off, the EEC and TFI are both asleep and won't be trying to do something bad.
With the ignition off, the EEC and TFI are both asleep and won't be trying to do something bad.
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From: Portage Twp, PA
What to do with spark when checking compression
I recently did a compression test on my '89 F-150 4.9L, and all I did was took out all the spark plugs (obviously, I took the wires off first). For an extra measure of EEC security, I disconnected the coil wire, so that in the unlikely event a plug wire was grounded, turning the key wouldn't fry anything.
The only reason the instructions go to all that detail of disconnecting things is to absolve them of any responsibility from damage. Pure stupidity.
One other thing, since we're on the topic...why do you HAVE to have the throttle open and the engine warm for a compression test? If you perform the test the same way for all cylinders (cold or hot, throttle open or closed), then the results should theoretically be the same. If not, why not?
Pat
The only reason the instructions go to all that detail of disconnecting things is to absolve them of any responsibility from damage. Pure stupidity.
One other thing, since we're on the topic...why do you HAVE to have the throttle open and the engine warm for a compression test? If you perform the test the same way for all cylinders (cold or hot, throttle open or closed), then the results should theoretically be the same. If not, why not?
Pat
Last edited by PatsPOS; Jun 15, 2003 at 07:31 AM.
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What to do with spark when checking compression
I hope you're not asking me, Pat, cause I don't have a clue. I took my plugs out last night and am planning to check the compression today so the engine is going to be cold as can be. If you are just comparing cylinders to see if they are all within 10% of each other it seems like it wouldn't matter, but the overall readings might be lower on a cold engine because the rings would expand when they were hot. I'm afraid as slow as I am to get the plugs out, if the engine was hot then it would be cold by the time they were all out anyway.
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What to do with spark when checking compression
I'm thinking the heated rings may expand, thus giving a better compression reading, plus if one is looking for headgasket cracks/leaks, the heated gasket's cracks may be more open?
I'm also thinking that the throttle should be open beacuse air is what you are attempting to compress, and then meausre, in the cylinder - if the throttle were completley closed off, and carbon gunk was blocking off any hopes of air to get in, you may end up with a lesser volume of air than the cylinder should have, thus the compression reading would be lower than it should be.
I would suppose that if you kept cranking, eventually the cylinder would get the air it needed to read correctly. But keeping the throttle open makes this happen faster.
If you are doing the test just to make sure the cylinders are all the same, then it probably won't matter how you do it. But if you are trying to compare all cylinders to what the compression reading should have been from the factory, the warm-engine method is the one they normally go by (i think).
Best,
I'm also thinking that the throttle should be open beacuse air is what you are attempting to compress, and then meausre, in the cylinder - if the throttle were completley closed off, and carbon gunk was blocking off any hopes of air to get in, you may end up with a lesser volume of air than the cylinder should have, thus the compression reading would be lower than it should be.
I would suppose that if you kept cranking, eventually the cylinder would get the air it needed to read correctly. But keeping the throttle open makes this happen faster.
If you are doing the test just to make sure the cylinders are all the same, then it probably won't matter how you do it. But if you are trying to compare all cylinders to what the compression reading should have been from the factory, the warm-engine method is the one they normally go by (i think).
Best,
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From: Live Oak, FL
What to do with spark when checking compression
As Ludis & Patspos suggested, I pulled the coil wire, left the key off and cranked it by shorting the starter selenoid.
Got it all back together and it still runs so I guess I didn't fry anything.
I've got two threads going on the same general subject. If you want to see the test results, look at LOOSING POWER GOING UP HILLS.
Thanks for all the help.
Got it all back together and it still runs so I guess I didn't fry anything.
I've got two threads going on the same general subject. If you want to see the test results, look at LOOSING POWER GOING UP HILLS.
Thanks for all the help.
